Tube rolling mill with a tapered mandrel

ABSTRACT

A TUBE ROLLING MILL HAVING TWO SETS OF THREE ROLLS EACH WHICH ARE RECIPROCATINGLY DRIVEN ALONG A LENGTH OF A TUBE SUPPORTED BY A TAPERED MANDREL. EACH ROLL IS FORCED AGAINST THE TUBE BY ITS INDIVIDUAL CAMS EACH HAVIN A SURFACE WITH ONE OR MORE TAPERS WHICH ARE RELATED TO MULTIPLE MANDREL TAPER IN A MANNER TO PROVIDE REDUCTION IN BOTH WALL THICKNESS AND INSIDE DIAMETER OF THE TUBE. EACH ROLL IS PROVIDED WITH A TUBE CONTACTING GROOVE HAVING IN CROSS-SECTION A CENTRAL ARC WITH A RADIUS OF CURVATURE SUBSTANTIALLY EQUAL TO THE SMALLEST RADIUS OF THAT PORTION OF THE TUBE CONTACTED BY THE ROLL, WITH EITHER SIDE OF THE CENTRAL ARC JOINED BY LINES TANGENT THERETO WITH LARGE RADII OF CURVATURE CHOSEN SO THAT EACH ROLL CONTACTS A TUBE IN TWO ZONES AROUND ITS CIRCUMFERENCE. THE ROLLS ARE PRESSED AGAINST THE TUBE UPON THE URGING OF THE TAPERED CAM SURFACE AGAINST A ROLL TRUNNION. THE RADIUS OF THE TRUNNION IS CAREFULLY CHOSEN TO CONTROL LONGITUDINAL FORCES TRANSFERRED TO THE TUBE BY THE ROLL.

Oct. 12, 1971 GABEL ETAL 3,611,775

TUBE ROLLING MILL WITH A TAPERED MANDREL Filed July 29, 1969 4 Sheets-Sheet 1 4 Sheets-Sheet I R. H. GABEL E AL Oct. 12, 1971 TUBE ROLLING MILL WITH A TAPERED MANDREL Filed July 29. 1969 4 Shoots-Sheet I5 x 1 H I O m Q 0 m l||||| mv 301 m0 muzqkwfi wxomkm T mm :01 m0 muzdPmE mxOmFm lllllllv mzON @ZUEOB mmsk Y Oct. 12, 1971 R.H.GABEL ETAL TUBE ROLLING MILL WITH A TAPERED MANDREL Filed July 29, 1969 R. H. GABEL ETA!- TUBE ROLLING MILL WITH A TAPERED MANDREL Oct. 12,1971

4 Sheets-Sheet 4 Filed July 29, 1969 DIRECTION OF ROLL TRAVEL United States Patent 3,611,775 TUBE ROLLING MILL WITH A TAPERED MANDREL Richard H. Gabel and Frank L. C. Williams, Norristown, Pa., assignors to Superior Tube Company, Norrlstown, P

Filed July 29, 1969, Ser. No. 845,833 Int. Cl. B21b 17/10 U.S. Cl. 72193 8 Claims ABSTRACT OF THE DISCLOSURE A tube rolling mill having two sets of three rolls each which are reciprocatingly driven along a length of a tube supported by a tapered mandrel. Each roll is forced against the tube by its individual cams each having a surface with one or more tapers which are related to multiple mandrel taper in a manner to provide reduction in both wall thickness and inside diameter of the tube. Each roll is provided with a tube contacting groove having in cross-section a central arc with a radius of curvature substantially equal to the smallest radius of that portion of the tube contacted by the roll, with either side of the central are joined by lines tangent thereto with large radii of curvature chosen so that each roll contacts a tube in two zones around its circumference. The rolls are pressed against the tube upon the urging of the tapered cam surface against a roll trunnion. The radius of the trunnion is carefully chosen to control longitudinal forces transferred to the tube by the roll.

CROSS-REFERENCE TO A RELATED APPLICATION This application is related to patent application Ser. No. 845,832, now abandonded by Richard E. Russell, entitled Tube Rolling Mill Employing a Tapered Mandrel and a Cluster of Rolls That Each Have Specially Designed Tube Contact Grooves, filed concurrently herewith.

BACKGROUND OF THE INVENTION This invention relates generally to a method and apparatus for reducing and elongating metal tubing and more particularly to improvements in a method and apparatus for reducing and elongating metal tubing with the use of rolls.

Metal tubing is used in a wide variety of environments and for many different applications. This requires that the tubing be available with a wide variety of inside and outside diameters and Wall thicknesses. In order to effectively utilize the economies of mass production, metal tubing is initially manufactured in only a few standard sizes. This makes it necessary to modify tubing of a standard manufactured size to obtain a size needed for a particular application involving smaller quantities of tubing than can be economically manufactured directly.

A machine for reducing tubing of a standard manufactured size is described by Krause in the Iron and Steel Engineer, August 1938, pages 16-29, and in several patent publications such as US. Pats. Nos. 2,161,064, 2,161,065, and 2,223,039. The machines described therein utilize a single set of two rolls operating against a mandrel supported tube to effect the tubes reduction. Each roll is rolled along a working length of the tube in response to the movement of a cam for controlling the pressures exerted by the roll against the tube. These machines provide for only a limited reduction in wall thickness without significant reduction of the inside diameter of the tube.

In addition, there have been several disclosures by Argonne National Laboratories relating to similar machines. In an Argonne paper ANLMS990, dated March 1968, entitled Fabrication of Vanadium Alloy Tubing,

Patented Oct. 12, 1971 "ice by Mayfield and Karasak, a rolling mill is disclosed which is capable of limited inside diameter tube reduction. In an Argonne print MYB-2700A19D, part 33D, a roll groove design is described wherein the roll groove has a center arcuate surface terminating in two tangent straight surfaces at its edge. The radius of the arcuate surface matches the longest radius of a tube length to be worked by the roll, whereby the tangent flat surfaces serve only as side relief for non-uniform deformation of the tube. Such single point contact of the roll groove with a tube is not an eflicient working arrangement.

Several publications by Russian authors have described similar tube rolling mills. These disclosures, however, tend to show that three or six roll tube rolling machines now in existence have merit in reducing wall thickness while posessing only limited capability for reducing the inside diameter. Such machines do not enjoy a wide application to the tube industry since the majority of such applications require substantial reduction in diameter.

Therefore, it is a primary object of this invention to provide a tube rolling mill capable of making large reductions in both wall thickness and inside diameter.

It is also an object of this invention to provide a tube rolling mill with a high efiiciency and feed rate.

It is a further object of this invention to provide a tube rolling mill with a high degree of reliability and additionally with a capability of producing reduced tubes with a wide variety of wall thicknesses and inside diameters.

SUMMARY OF THE INVENTION These and additional objects are accomplished by a machine according to this invention in which a number of elements are combined to reduce tube wall thickness and additionally reduce the tube inside diameter without a machine complexity any greater than exists in other tube reducing machines. Two sets of three rolls each are provided for contacting a tube with grooves provided around the circumferential edges of each roll. Each roll is rotatably attached to a roll housing and is reciprocated thereby along a working length of a mandrel which is designed to fit within a tube to be reduced. Each set of three rolls is clustered around the tube with their axes of rotation located in a plane substantially perpendicular to the length of the mandrel and displaced from each other. Two sets of rolls are spatially fixed relative to one another in a direction along the length of the mandrel by the roll housing. One set of rolls reduces the tube an intermediate amount and the second set of rolls completes the reduction to the dimensions desired. The axes of rotation of the rolls of one set are displaced 60 from the axes of rotation of the rolls of the other set. The mandrel is tapered along at least a portion of its working length over which the roll housing reciprocates the tube contacting rolls. The rolls are urged toward the mandrel by an individual cam surface for each roll, said cam making a contact with a roll along its surface substantially opposite the tube contacting surface. The cam and mandrel are shaped so that each roll contacts a tube to be reduced during at least a portion of each reciprocating stroke of the roll housing.

The tube contacting groove around the outer edge of each roll has a uniform shape in cross-section at any point around the circumference of the roll. As each roll is pressed against a tube and rolled therealong, the outside diameter of the tube becomes smaller. The cross-sectional shape of the groove includes an arcuate portion in the middle thereof having a radius of curvature substantially equal to the smallest radius of the outside surface of a tube portion contacted by that particular roll. On either side of this arcuate portion of the groove is in crosssection a substantially flat portion extending on either side to the edge of the groove, the flat portions joining the arcuate portion as a tangent thereto. The advantage of this arrangement is that at the beginning of the working stroke, a tube contacts the roll groove at two portions (zones) therealong and does so throughout the working stroke until the tube is reduced to an outside radius substantially equal to the radius of curvature of the arcuate portion of the roll groove. The two-zone contact of each roll groove makes more efficient use of the working stroke. With a six roll machine, a tube is contacted at twelve zones therearound, thereby providing considerably more surface area contact between the rolls and the tube than in other machines. This results in a greater effective volume of metal reduction per rolling stroke and a better quality reduced tube.

The cams and the mandrel are cooperatively shaped in a manner to provide a maximum bite of each roll at its contact zones into the tube to be reduced. A maximum bite is provided throughout wall reduction portions of the working stroke and is slightly less than the bite which will strain any portion of the tube to its point of rupture. At the beginning of the working stroke, this bite is much larger than at the end of the working stroke. This maximum bite at points along the tube is dependent upon, among other things, the material of the tube being reduced. Such a maximization of the roll bite additionally improves the reduction possible in a given working length of the machine.

The cams are attached to a cam housing which is reciprocated along the length of the mandrel in order to contact the rolls and guide them against the tube without significant slippage between each roll and its cams. Instead of contacting the outer circumference of the roll, the cams are designed to contact trunnions provided on either side of each roll. This has the advantage that a larger bearing surface may be provided to prolong the life of the rolls and the cams, as well as to allow an adjustment which is beneficial to the tube rolling process. The roll housing and cam housing are driven from a common motor source with a constant ratio of velocities therebetween. This velocity ratio and the relationship between the radius of the rolls trunnions and the radius of that portion of the roll groove contacting a tube at a specific point thereof determines the degree to which the roll will tend to apply torque to the tube and cams, and thereby determines the tendency of the groove to slip against the surface of the tube when the tube is adequately held against lateral movement. For a given configuration, the torque forces causing such slippage vary along the working length of the tube since the effective radius of the roll varies therealong. It has been found desirable to control the torque forces tending to slip the roll against the tube in a manner to compensate for the horizontal forces applied to the tube by a roll biting into the tube wall. Such compensation allows mandrel and tube gripping devices to more accurately position the tube. The compensation also allows larger roll bites, thereby accomplishing more tube reduction in a given working length.

For a more detailed understanding of the invention and for further objects and advantages thereof, reference is made to the following description of its preferred embodiments taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates in a partially exploded view a rolling mill according to the present invention;

FIG. 2 is a cross-sectional view of FIG. 1 taken through the first stage (set) of rolls at 22;

FIG. 3 is a cross-sectional view of FIG. 1 taken through the second stage (set) of rolls at 33;

FIG. 4 schematically illustrates the operation of the primary operating components of the rolling mill illustrated in FIGS. 1, 2 and 3;

FIG. 5 shows the shape of a preferred tube contacting groove of a roll for use in the rolling mill shown in FIGS. 1-3;

FIG. 6 shows a side view of a preferred roll illustrating an optimum trunnion radius for use in the rolling mill shown in FIGS. 13;

FIG. 6A shows the roll of FIG. 6 performing a modified type of work on a tube.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1, a cam housing 11 is reciprocated relative to a machine frame 13 along a slide 15 in substantially a straight line. An electric motor 17, also attached to the frame 13, drives a flywheel 19. A rod 21 (partially broken away) is connected between the fiy wheel 19 at a crank pin 20 and the cam housing 11 to convert rotary motion of the flywheel to reciprocable motion of the cam housing. Within the cam housing 11 is a reciprocatable roll housing 23, shown removed from the cam housing for clarity of illustration. A pinion gear 25 engages a rack 27 that is rigidly attached to the frame 13. A second pinion gear 26 engages a rack 29 that is rigidly attached to the cam housing 11. The pinion gears 25 and 26 are concentric about a common axis of rotation 30 and are non-rotatable relative to each other. The reciprocable motion of the axis of rotation 30 of the pinion gear 25 is communicated to the roll housing 23 by means of a connecting rod 31 (shown herein as two sections since the roll housing 23 is shown removed from the cam housing 11). The cam housing 11 has a maximum reciprocation stroke distance that is equal to the diameter of the circular path taken by the crank pin 20. From the geometry of the driving arrangement of FIG. 1, the roll housing 23 has a maximum reciprocation stroke distance that is equal to the maximum stroke of the cam housing 11 multiplied by the diameter of the pinion gear 25 and then divided by the sum of the diameters of the pinion gears 25 and 26. The use of two pinion gears having different radii as herein has the effect of increasing the length of the working zone along the tube without increasing the stroke length of the cam housing. It should be noted that although the double pinion gear arrangement herein is very convenient for controlling the maximum relative cam housing and roll housing stroke distances, and thereby their relative velocities, other specific mechanical arrangements, such as one employing levers, may also be employed for the same purpose.

Another aspect of the geometry of the arrangement in FIG. 1 is that the cam housing stroke distance is equal to the sum of the roll housing stroke and the working stroke length of the cams (the distance along each cam that contacts a roll trunnion) within the cam housing. It follows, then, that the cam length contacted by each roll bears the same relationship to the roll housing stroke as a ratio of the diameter of the cam housing pinion gear 26 to the diameter of the roll housing pinion gear 25.

A tube 33 to be reduced is inserted through an opening 35 of the roll housing 23, and is carried by a mandrel 37. The mandrel 37 is rigidly held fixed relative to the machine frame 13 by an appropriate gripping device 36, which also provides for removing the mandrel. An appropriate apparatus 38 is provided for positively gripping the tube 33 and advancing it linearly over a working length of the mandrel 37, and further to incrementally rotate the tube. The apparatus 38 causes such feeding and rotation of the tube being reduced at specific portions of the reciprocating cycle, as in hereinafter discussed.

FIGS. 2 and 3 better show the relationship of tube deforming rolls and the cam housing as sectional views of FIG. 1. A first set of rolls 39, 41 and 43, shown in FIG. 2, are held in the roll housing 23 with their axes of rotation lying substantially in a plane perpendicular to the mandrel 37 and making an angle of with each other. Similarly, a second set of rolls 45, 47 and 49, shown in FIG. 3, are held by the roll housing 23 in the position that their axes of rotation lie substantially in a plane perpendicular to the mandrel 37 and at a spatially fixed distance along the length of the mandrel from the plane in which the axes of rotation of the first set of rolls 39, 41 and 43 lie. Furthermore, the axes of rotation of the two sets of rolls are angularly displaced from each other by 60.

Although held by the roll housing 23 against movement relative thereto in the direction of its reciprocation, each roll is free to move in a direction normal to the mandrel. Each roll is resiliently urged by a set of springs (such as springs 48 and 50, each held within a roll guide) out of the roll housing 23 and against its associated cam surfaces. Alternatively, the rolls may be hydraulically urged against their associated cams. Each roll has a trunnion formed on either side thereof, such as trunnions 51 and 53 on either side of the roll 45, Each roll is associated with a pair of cam tracks upon which its pair of trunnions ride. The two cams associated with each roll are designated herein with the same number as the roll but with an asterisk placed after the reference number of one of the cam tracks and a double asterisk placed after the number referring to the other of the cam tracks. The cams are long metal bars shaped in a manner discussed hereinafter and rigidly attached to the cam housing 11. This attachment is accomplished through a recessed member for each pair of cams, such as a recessed member 55 which is shaped to support the earns 45* and 45*? Notice that the cams 45* and 45" each have a sloped side surface which allows fastening them to the recessed member 55 by a wedge 57 which is attached to the recessed member by a threaded fastener 58.

It should be noted with reference to FIGS. 1, 2 and 3, the ease with which the cam surfaces may be replaced in the cam housing 11 and also the ease with which the rolls may be replaced in the roll housing 23. A given pair of cams are removed by removing their associated wedge. The rolls are merely lifted out of the roll housing 23 when the roll housing is removed to a position as illustrated which is out of the cam housing 11. The mandrel 37 is also easily removed. These features allow quick conversion of the tube rolling mill to receive raw tubes of various sizes and also to produce finished tubes with various wall thicknesses and inside diameters.

The schematic diagram of FIG. 4 illustrates operation of the rolling mill illustrated in FIGS. 1, 2 and 3. The mandrel 37 is shown in cross-section along its length which includes a tube working zone BJ wherein at all points therealong the tube 33 is contacted by one or both sets of rolls to accomplish reduction either in wall thickness or inside diameter or both. The mandrel has a diameter d at its large end which is something slightly less than the inside diameter of the starting tube 33, thereby allowing the tube to be slid easily over the mandrel. The small end of the mandrel has a diameter (1 which is substantially equal to the desired inside diameter of the reduced tube. The mandrel 37 is gradually tapered within the tube working zone from one of these diameters to the other. This taper is significantly in excess of that required for tube relief. The diameters d and d may differ by 20% or 30% or more, depending on the tube inside diameter reduction desired.

In order to demonstrate the cooperation between the cams and the mandrel, one roll from each of the two sets of rolls is shown in FIG. 4 as if they operated in the same plane so that the relationship between them and their cooperation in reducing the tube are illustrated. Rolls 39 and 45 are illustrated in FIG. 4 along with their associated cams 39' and 45*, respectively. The axis of the roll 39 reciprocates along the tube between positions A and I with a distance therebetween equal to the stroke distance of the roll housing 23 (not shown in FIG. 4) in which the roll 39 is journaled. Similarly, the axis of the roll 45 reciprocates along the tube between positions D and K. The earns 39* and 45* are attached to the cam housing 11 (not shown in FIG. 4) and thereby are reciprocatably driven at a greater velocity than the axis of the 6 rolls, as described hereinabove. The cam 39* contacts a trunnion 40 attached to the roll 39 and the cam 45* contacts the trunnion 51 of the roll 45. The shape of the cams and of the mandrel determine the displacements of rolls downward against the tube to bring about a desired deformation of the tube.

Consider a single working stroke wherein the rolls and cams of FIG. 4 move from their far left hand position to the far right and back again. This represents the extent of movement brought about by a single revolution of the flywheel 19 of FIG. 1. The roll 39 begins at the position A and the roll 45 begins at the position D. As shown by the dashed lines, the roll 39 contacts the tube 33 for the first time at about the position B and the roll 45 contacts the tube 33 for the first time at approximately the position F. Proceeding further to the right, the cooperative shapes of the cams and the mandrel allow the roll 39 tobe lifted from the tube 33 at about the position F, as shown by the path [39] of the roll, away from the tube. Similarly, the roll 45 is caused to be lifted from the tube 33 at about the position I, as shown by the path [45] of the roll, away from the tube. The roll 39 arrives at the position I at the same time the roll 45 arrives at the position K to complete the first one-half of the working stroke. The rolls 39 and 45 then move back to their beginning positions A and D, respectively, to complete one working stroke cycle. It may be noted that the cam working length as used herein is a horizontal projection of the length of a cam surface contacted by the trunnion. With reference to the cam 39*, the cam working length thereof is the horizontal distance between points A and I The tube 33 is advanced (fed) by the apparatus 38 an increment to the right while the rolls are drawn away from contact with the tube, either at one or both ends of the working stroke. Similarly, the apparatus 38 rotates the tube 33 either at one or both ends of the working stroke. It is preferred to rotate the tube through a small angle at each end of the working stroke because a smoother finished tube is the result. The shape of the tube 33 shown in FIG. 4 within the working zone represents the finished shape thereof after a working stroke and before the tube is fed an increment in preparation for the next working stroke.

There are many specific cam and mandrel shapes that may be utilized depending upon the specific tube reduction desired. FIG. 4 illustrates a preferred arrangment for major inside diameter reduction. The following tabulation describes the work done by the roll 39 within the working zone between lettered positions along the length of the tube:

Between B-C: The tube is reduced to intimate contact with the mandrel.

Between C-E: Primarily tube diameter reduction is accomplished by the roll 39.

Between E-F: Primarily wall reduction is accomplished by the roll 39.

The following tabulation describes the work concurrently performed by the roll 45 within the working zone between lettered positions along the length of the tube:

Between F-G: Primarily wall reduction performed by the roll 45.

Between G-H: Primarily wall reduction performed by the roll 45 but with a lesser bite into the tube than between FG.

Between H-J This is a finishing zone where there is substantially no taper to the mandrel 37 and with very little bite of the roll into the tube.

To accomplish the above-noted specific tube reductions at various points within the tube working zone, the man.- drel has one or more straight line tapers. The cams are shaped cooperatively therewith, each having a plurality of straight line tapers. The cams of FIG. 4 have their roll containing surfaces marked with subscripted letters corresponding to the lettered positions along the tube. For example, when the roll 39 is positioned at E along the tube, the cam 39* is contacting the trunnion at position E Straight line tapers are preferred for the cams and the mandrel since they are easy to machine, although continuous curves may also be employed.

The description herein with respect to FIG. 4 is exemplary only with various changes in the specifics thereof being possible. For example, if major inside tube diameter reduction is not required, the portion B -F of the cam 39* may be shaped differently relative to the portion BF of the mandrel than as shown to effect tube wall reduction between B-F instead of tube diameter reduction. Also, the elements may be designed so that the rolls 39 and 45 overlap in their work zones along a portion of the tube, preferably with dissimilar cam tapers acting on the two rolls in this common length of the tube. Also, cer tain applications may require only a single taper along a working length of each of one set of cams. Furthermore, in those cases where little inside diameter reduction is desired, the cams and rolls described herein may be used with a mandrel having little or no taper.

Along any of the portions of tube length wherein substantial wall thickness reduction is desired, the controlling cam and mandrel tapers are designed for a bite of the rolls into the tube at each point within this portion that is approximately the same percentage of the wall thickness at that point before the roll. This maximizes the efficiency of the wall thickness reduction, thereby allowing more reduction to be accomplished in a shorter portion of the working stroke. Multiple straight line tapers on the cams may be employed to approximate this constant percentage although continuous curved cam surfaces are more exact. The amount of tube feed for each stroke is then adjusted to a maximum for a given tube material just short of that which ruptures the tube, thereby maximizing productivity of the machine.

A preferred tube contacting groove is illustrated in FIG. 5 for the rolls of a rolling mill illustrated with respect to FIGS. 13. The groove shape is uniform in cross-section at any radial plane thereof. The groove crosssection is shown on a roll 79 which represents relative roll groove dimensions for any roll shown in FIGS. 1-3 for the purpose of describing roll groove design. In the center of the groove is an arcuate portion 81 having a center of curvature at a point 83. Joining either side of the arcuate center portion 81 as tangents thereto at its end points 85 and 87 are straight line segments 89 and 91 which extend to the groove outside edges 97 and 99, respectively. The arcuate portion 81 extends for an angular distance (1: on either side of a center line.

The radius of curvature of the arcuate portion 81 is made substantially equal to the smallest outside tube radius the roll is designed to contact, represented by a circle 93. This would be the radius of the finished tube for the roll 45 shown in FIG. 4 and the radius of the tube at location F for the roll 39. A circle 95 represents the largest outside tube radius which the roll groove is designed to contact, that of the beginning tube for the roll 39 and that of the tube at position F for the roll 45 in FIG. 4.

This roll groove design provides two zones of contact for each roll against the outside of the tube between the tubes larger portion (95) and substantially until its smallest portion (93). Such two-zone rolling accomplishes more reduction in a given working zone of a tube when compared to a roll groove providing only one zone of contact with the tube. Non-uniform tube wall strain is reduced as well as resulting degradation of the tube quality. Also, required rolling forces, and thus machine wear, are reduced. To optimize these advantages, the radius of the arcuate center portion 81 of the roll groove may be made 1 or 2 percent less than the smallest outside tube radius to be contacted by the roll groove, thereby extending two zone rolling over the entire length of the tube contacted by the roll whereby roll life is extended. An arcuate center portion 81 with a radius significantly smaller than the finished tube outside radius (in the extreme the groove becomes V-shaped) results in a finished reduced tube surface that is irregular and rough.

When the roll 79 of FIG. 5, is pressed against a tube during the rolling thereof, it bits into the tube at its zones of contact with the roll groove. This results in the extreme edges 97 and 99 becoming closer to the tube. It is desirable to maintain a clearance between the outside edges 97 and 99 and the tube outside wall to prevent scoring or grooving of the tube. Therefore, a roll having a given groove is limited to a maximum tube bite of something less than the distance between the groove edges 97 and 99 and the largest tube portion to be contacted by the roll, a distance g shown in FIG. 5. This clearance of the edges 97 and 99 is increased by decreasing the angle However, as o is reduced to small values, the two zones of tube contact become closer together thereby increasing wall strain and the possibility of localized tube failure. Therefore, there is a trade-off between the desired to maximize the bite of the roll into the tube and the desire to maintain the advantage of two zone rolling. For a given tube material, there is an optimum angle which allows the roll to take the most efiicient maximum bite into the tube, thereby resulting in the most rapid feed rate of the tube through the machine. An angle of from 30 to 38 degrees is satisfactory for most common tube materials and specific types of reduction.

The tangential portions 89 and 91 of the roll groove are shown in FIG. 5 as straight lines. However, these portions of the groove may, alternatively, be given a curvature with one or more finite radii of curvature. The edges 97 and 99 of the roll groove may be curved to eliminate the sharp corner which can damage the tube surface. The remaining segments of the tangential portions 89 and 91 may then remain straight or may be curved slightly, either concave or convex. If part of the tangential portions is made concave, tube wall shear decreases further since the total area of tube contact becomes larger. However, the edges of the groove are then brought closer to the tube outside wall by such a concave shape which places limits on the maximum tube bite that may be taken by such a roll groove. If part of the tangential groove is made convex, the groove edges are removed further from the tube wall thereby allowing an increased tube bite without the edges of the groove scoring the tube surface.

It may be noticed from FIG. 5 that as the roll 79 is driven from the largest tube outside size to the smallest size 93 and back again to complete a single working stroke, the rolling radius of the roll 79 changes from (max to r and back to r again. However as an aid in describing an optimum trunnion radius r on a roll such as that shown by its side view in FIG. 6, assume initially that the roll radius contacting a tube 100 is a contact r throughout the working stroke. A trunnion 101 is contacted by its associated cam at a point 103 and the cam imparts a downward force thereon plus a horizontal frictional force, as shown by the arrows in FIG. 6 at the point 103. The roll is also subjected to restraining forces at its center 107 by the roll housing in which the roll is journaled. Assume further that the bite B of the roll into the tube is substantially equal to zero, as shown in the FIG. 6A modification of the general case shown in FIG. 6. The amount of bite B at any particular position along the tube is determined, as discussed hereinabove with respect to FIG. 4, by the relative shapes of the mandrel and the roll cams. Referring to FIG. 6A, a tube 100 is contacted at a roll point 104, exerting thereon a downward force and possibly a tangential frictional force. In this simplified situation, it is desirable to minimize the tangential force (torque force) transmitted to the tube by rotation of the roll thereon. By minimizing the torque force applied by the roll to the tube, the tube is more easily held in place against oscillatory movement along its length and reduced machine stress is accomplished at a number of locations.

The torque force is minimized by careful selection of the trunnion radius r The torque force applied to the tube 100 is zero when the roll groove contacts the tube with a zero velocity relative thereto as the roll proceeds along the length of the tube. This zero velocity condition is met when the following expression is satisfied:

roll housing stroke 1;,

cam Working length r (1) For the particular roll housing and cam housing driving arrangement shown in FIG. 1,

roll housing stroke cam working length diameter of pinion gear 25 diarneter of pinion gear 26 (2) The quantity N is defined in Equation 2 as a convenience so the relationship between a trunnion radius r,; and a troll radius r may be expressed as follows to obtain a condition of zero torque force applied to the tube by the roll:

The quantity Nr is referred to herein as the effective roll trunnion radius.

If the effective trunnion radius is less than the roll radius r the tube is driven in a direction opposite to that taken by the roll along its length by a torque applied by the roll. Conversely, if the effective trunnion radius is greater than r,,, the tube is driven by the torque in the same direction in which the roll is traveling.

It has been assumed in the discussion hereinabove with respect to FIG. 6A that the roll is taking substantially no bite into the tube being rolled. Referring again to FIG. 6, the roll is illustrated as biting into the wall of the tube 100 an amount B by contact along the line 105 of the roll groove. It will be noticed that such tube rolling produces a horizontal component of force h at the roll contact portion 105 which tends to drive the tube 100 in the same direction in which the roll is traveling. If the roll reciprocates back and forth over the tube, these forces alone tend to establish oscillatory motion in the tube 100 along its length. Such oscillatory motion is undesirable because of added tool stresses created thereby. The torque forces which may be applied through the roll to the tube, as described hereinabove with (respect to FIG. 6A, may be utilized to compensate for the horizontal force It imparted to the tube as a result of a bite B therein. To accomplish such compensation, the effective trunnion radius is made less than the roll radius r in order for a roll torque force to be produced which tends to drive the tube 100 in a direction along its length opposite to that of the rolling direction. The greater the size of the bite B, the smaller the effective trunnion radius would be made. Furthermore, as larger bites are taken into the tube in order to increase productivity, the horizontal force h created thereby on the tube 100 increases and the need for compensation to prevent excessive machine element stresses becomes greater.

The discussion hereinbefore with respect to FIGS. 6 and 6A has not considered the fact that the tube material is flowing with respect to the mandrel under the influence of the rolls Working either a Wall thickness or inside diameter reduction, or both. This is the desired metal flow which results in the tubes elongation and thus reduction in size. If a tube rolling mill utilizes a single roll or set of rolls, the effective trunnion radius Nr would be made slightly higher than that described above. This adjustment of the effective trunnion radius then results in the tube contacting points of the roll groove traveling at a zero velocity relative to a tube surface which itself is moving slightly.

If two sets of rolls, as described herein with respect to FIGS. l4 are utilized, there is the additional consideration that although the centers of each of the rolls are driven at the same velocity relative to the mandrel by common connection with a single roll housing, the tube portions independently contacted by each of the two sets of rolls may be moving at a different velocity relative to the tube mandrel since each set of rolls is accomplishing different work on the tube. Therefore, the effective roll trunnion radius of one set is also chosen relative to the effective trunnion radius of the other set in order to take into account the different flow velocity of the tube surface areas contacted by each of the roll sets.

It has been assumed from the discussions with respect to FIGS. 6 and 6A that the roll radius remains a constant r throughout the rolling mills working stroke. This is not, however, the case when the preferred roll groove as illustrated in FIG. 5 is utilized. The rolling radius varies between r and r,,,,,,,, as described hcreinabove with respect to FIG. 5. Therefore, the undesirable forces cannot be compensated exactly by roll trunnion design but may be minimized by appropriate effective trunnion radii selection according to the factors discussed hereinabove in order to minimize forces acting on the tube 100, and thereby minimizing stresses on other tool elements to increase their wear and thereby improve the effectiveness of a rolling mill by reducing downtime because of tool failures.

For the rolling mill example described with respect to FIGS. 1-4, the set of rolls illustrated by the roll 39 in FIG. 4 preferably each have an effective trunnion radius substantially within a range of r to /2 (r +r The trunnions of these rolls are made as small as possible within this range. As the amount of diameter reduction accomplished by these rolls decreases, the effective trunnion radius is made smaller.

On the other hand, the effective trunnion radius of each of the set of rolls represented by the roll 45 of FIG. 4 is made as large as possible within substantially the same range of r to /2 (r +r As the amount of wall reduction accomplished by these rolls decreases, the effective trunnion radius is made larger but remaining substantially within this range.

What is claimed is:

1. A tube rolling mill, comprising:

a roll housing capable of being reciprocated along a straight line over a given working length,

at least one set of three rolls rotatably attached to said roll housing in a manner that the axes of rotation of said rolls lie in substantially a plane which is perpendicular to the path of reciprocation, said axes displaced from one another substantially in said plane, each of said rolls having a tube contacting groove of uniform cross-sectional shape around an outer circumferential surface. thereof,

an elongated mandrel of circular cross-section extending along said line of reciprocation and through an opening of said roll housing,

said mandrel characterized by a first length having a first substantially uniform diameter therealong, a second length having a second substantially uniform diameter therealong, and a tapered transitional section joining said first and second lengths, said first diameter being significantly greater than said second diameter, the working length of said mandrel over which a tube is rolled including substantially the entire tapered transitional section plus an adjoining small portion of said second length, thereby providing for reducing the inside diameter of a tube from a diameter equal to or greater than said first diameter to a finished inside diameter substantially equal to said second diameter, and

a cam for each of said rolls positioned to guide said roll within a predetermined distance of said mandrel along the working length, said cams being attached to the inner surface of a cylindrical cam housing which surrounds said roll housing and which is also capable of reciprocation in a direction of said straight line.

2. A tube rolling mill according to claim 1 wherein the tapered transitional section of said mandrel includes a plurality of adjacent tapers each with a distinct and uniform slope.

3. A tube rolling mill according to claim 1 wherein said cams are shaped to allow withdrawing of the rolls from contact with a tube at each end of a reciprocation stroke of said roll housing and cam housing.

4. A tube rolling mill according to claim 1 wherein said first diameter and said second diameter of said mandrel differ by 20 percent or more.

5. A rolling mill, for reducing the inside diameter of tubing, comprising:

an elongated mandrel of circular cross-section adapted for insertion within a hollow circular tube to be reduced, said mandrel having a transition length in which its cross-sectional size changes from a first diameter at one end of the transition to a second diameter at another end of the transition, said transition length having a taper significantly in excess of that on either side of said transition, said second diameter being significantly less than said first diameter,

a roll housing surrounding the mandrel and adapted for reciprocation along a length of said mandrel that includes said transition length,

at least one set of three rolls rotatably attached to said roll housing with the axes of rotation of said rolls laying in substantially a plane that is perpendicular to the mandrel, said axes being displaced from one another by substantially 120 in said plane,

each of said rolls having a tube contacting groove of uniform cross-sectional shape around its outer circumferential surface, said at least one set of three rolls additionally being attached to said roll housing so that each roll is free to roll along a tube supported by the mandrel when said roll housing is reciprocated,

a cam housing surrounding said roll housing and adapted for reciprocation along a length of said mandrel that includes said transition length,

means for reciprocating said roll housing and said cam housing along the mandrel so that said at least one 12 set of three rolls travels along a tube over said mandrel transition length and an adjoining mandrel portion on the second diameter side of the transition, and

at least one cam attached to an inner surface of said cam housing for each of said rolls, each cam being shaped and positioned to guide its associated roll along a predetermined path relative to the mandrel when the roll housing and the cam housing are reciprocated, said predetermined path chosen to force the tube to conform with the shape of the mandrel along substantially its entire transition length and its said adjoining portion on the second diameter side of the transition.

6. A rolling mill according to claim 5 wherein each cam is additionally shaped and positioned to guide its associated roll along a predetermined path relative to the mandrel that reduces wall thickness of a tube as well as its inside diameter.

7. A tube rolling mill according to claim 5 wherein the transition length of said mandrel includes a plurality of adjacent tapers, each of said plurality of tapers being of a uniform slope that is distinct from the other of said plurality of tapers and that is greater than the slope of any taper of the mandrel on either side of said transition.

8. A tube rolling mill according to claim 5 wherein said first and second diameters differ by 20 percent or more.

References Cited UNITED STATES PATENTS 1,890,803 12/1932 Coe 72-208 2,041,937 5/1936 Korbuly 72--209 2,161,065 6/1939 Krause '72209 2,388,251 11/1945 COe 72208 2,713,801 7/1955 Singer et al 72220 2,914,973 12/1959 Crane et al 72-208 3,416,346 12/1968 Arrington 72-189 MILTON S. MEHR, Primary Examiner U.S. C1. X.R. 72209 

